Dimensions of a printed circuit board can be reduced by using an element built-in technique and a multi-layered technique. Further, the size of the entire electric device also can be reduced.
As shown in FIG. 6, when a printed circuit board 1 includes a large heat generation electric element 2, which consumes a large amount of electricity and generates a large amount of heat, if the dimensions of the printed circuit board 1 are minimized, the operation specifications of a small heat generation electric element 3, 4, 5 may be not satisfied. Here, the small heat generation electric element 3, 4, 5 consumes a small amount of electricity and has a low proper operation temperature limit. The large heat generation electric element 2 consumes a large amount of electricity and has a high proper operation temperature limit. The heat generated in the large heat generation electric element 2 affects the small heat generation electric element 3, 4, 5 by heat conduction of the printed circuit board 1. Therefore, the temperature may exceed the low proper operation temperature limit of the small heat generation electric element 3, 4, 5.
In view of the above difficulty, it is necessary to have a large heat radiation fin for radiating heat from the large heat generation electric element 2. Thus, the dimensions of the device increase. Further, as shown in FIG. 7, when a cooling fan 7 is formed on a casing 6, it is necessary to increase air blow volume of the fan 7. In this case, noise from the fan 7 may cause difficulty. The fan may generate an operation noise. Further, when the small heat generation electric element 3, 4, 5 is disposed on a downstream side of the large heat generation electric element 2, heat convection from the large heat generation electric element 2 and/or heat radiation from the large heat generation electric element 2 may affect the small heat generation electric element 3, 4, 5.
Thus, the printed circuit board 1 may be divided into two independent boards, so that the large heat generation electric element 2 is mounted on one independent board, and the small heat generation electric element 3, 4, 5 is mounted on the other independent board. Thus, the heat from the large heat generation electric element 2 does not affect the small heat generation electric element 3, 4, 5. However, in this case, when it is necessary to connect between the large heat generation electric element 2 and the small heat generation electric element 3, 4, 5 in order to communicate therebetween with high speed, impedance mismatch may be caused by a contact resistance when a connector connects the large heat generation electric element 2 and the small heat generation electric element 3, 4, 5.
Thus, it is required to connect between the large heat generation electric element 2 and the small heat generation electric element 3, 4, 5 with high speed communication.